Those bigger motors, 250's and larger, don't turn up the same kind of RPM that a little 125 or 100 or 80 do. So they can get away with less oil. When I was little younger, I rode a KX250 for 2 years at a 40:1 mix. Never a problem. I also rode a KDX200 for 3 years at 40:1. Also never a problem. But.....I have also always let all my bikes warm up completely before ever taking off on the trail too. That DOES make a big difference in the life span of a motor.
On the oil subject: The less oil you put in your gas means the more gas you have entering the cubustion area. The more gas you have, the richer it will run. Oil is not really considered a burnable item. Gas and air is. So if you want to think of it like this, take oil out of the equation and just look at it as two parts, gas and air. If you ad more oil, you are putting less burnable gas into the motor. So it will run leaner.
If you put to much oil in the gas, you are decreasing the amount of gas and the ratio of gas to air can become to much air to fuel. This means a lean condition.
Or you could just take my word for it. To much oil can result in a lean condition if the motor is not loading up from to much oil first. To little oil can result in a rich condition if your motor does not lock up first from lack of oil.